This invention relates to instruments for measuring cathodic protection voltage levels and to the method used by such instruments to measure cathodic protection voltages.
Cathodic protection of metallic structures immersed in an electrolyte is a well established technique for reducing the rate of corrosion of the structure. An electric current is made to flow from the electrolyte to the structure and the efficacy of the protection is proportional to the current density at the surface of the structure. The protected surface is called the cathode and the surface from which the current flows into the electrolyte is called the anode. Cathodic protection is achieved at the cost of energy used to force the cathodic current to flow from the anode to the cathode through the electrolyte. Energy sources used to provide the cathodic current may take the form of sacrificial anodes or line powered AC to DC rectifiers. Distributing this current over the surface of the structure in a uniform manner to provide ample protection is an objective in the design and installation phase of any cathodic protection system.
Once installed, an indication of the adequacy of protection resulting from the operation of the cathodic protection system may be made by measuring the potential between the structure and the electrolyte. For each combination of metal and electrolyte, there is a potential that is deemed sufficient to ensure that no corrosion will occur. It is the determination of the true electric potential between the metal and the electrolyte with no errors or offsets that has proven to be a difficult task. Often, for structures buried in the earth, it is not feasible to expose the structure to make potential measurements. Measurements made some distance removed from the structure have been accepted as equivalent to the desired close measurements. The electrolyte in which the structure is immersed may be comprised of nearly dry earth to nearly all water and may vary from location to location and with climatic changes at a given location.
If a voltmeter is connected between the test point and a standard copper sulfate half cell situated on the surface of the earth directly above the pipeline, a voltage comprised of two components will be measured. The first component is the desired potential at the surface of the pipeline. The second is the voltage arising as some component of the cathodic protection current flows in the electrolyte in the earth. For a newly installed pipe which has not been subjected to cathodic protection, the potential at the surface of the pipe may be measured to be about 0.45 volts, depending upon the composition of the soil, the pipe, and the electrolyte in which they are immersed. After some months of cathodic current flow, the potential at the surface of the pipe may rise to about 0.80 volts as read on a voltmeter with the cathodic current temporarily interrupted to facilitate the measurement. The value of the soil resistance is unknown and variable with the season, soil conditions, and distance between the test point and the location of the half cell. In addition, the magnitude of the current component which flows through the soil is unknown and variable. As expected, the resulting IR drop cannot be anticipated. These IR voltages commonly exceed several volts and tens of volts in rare situations.
Although a copper sulfate half cell electrode has been most commonly used to ensure that a reproducible and stable contact might be made to the electrolyte during voltage measurements, other stable electrodes may serve equally as well. To further ensure cell accuracy, the current drawn by the measuring device through the internal resistances of the half cell should be minimized.
None of the many techniques for measuring the electrolyte potential, or what is sometimes called the IR drop free potential of a cathodically protected structure, has satisfied all the requirements of accuracy, simplicity and reliability.,. Most techniques rely upon the fact that the cathodic current is typically pulsating rather than constant. Some techniques even cause the interrupting of the cathodic current flow to create a pulsating current. There is a need for an instrument for measuring cathodic protection voltage levels which uses a simple voltage measuring probe that minimizes the effects of IR voltage drops.